Method of inserting fuel rods into individual cells in a fuel element grid

ABSTRACT

An illustrative embodiment of the invention provides a fuel element assembly technique that facilitates the insertion of a fuel rod into a cell which is formed by a resilient grid structure. Initially, a cam is introduced into the cell. The inserted cam is rotated to deflect the grid surfaces that comprise the cell walls. This deflection prevents a leaf-spring mounted detent formed in the grid surface from extending into the path of the fuel rod. A key with a dimension that generally matches the deflected grid is inserted into the cell in order to engage the temporarily bowed grid surface. The inserted cam is rotated back to the initial orientation relative to the cell configuration and is withdrawn from the grid structure. The deflecting key lodged against the bowed grid surface, however, sustains the deflection in order to enable the fuel rod to be inserted into the cell without being abraded by the detent. After fuel rod insertion, the matching dimension portion of the deflecting key is disengaged from the grid surface and the key is withdrawn from the cell to release the leaf spring and allow the detent to clutch the fuel rod.

Jabsen METHOD OF INSERTING FUEL RODS INTO INDIVIDUAL CELLS IN A FUELELEMENT GRID [75] Inventor: Felix S. Jabsen, Lynchburg, Va.

[73] Assignee: The Babcock & Wilcox Company, New York, NY.

[22] Filed: Oct. 28, 1971 [21] Appl. No.: 193,383

[52] US. Cl 29/433, 29/235, 29/241, 29/450, 176/78 [51] Int. Cl 132319/04 [58] Field of Search..... 29/433, 446, 450, 241, 235; 176/78 [56]References Cited UNITED STATES PATENTS 3,255,091 6/1966 Frisch 176/783,379,617 4/1968 Andrews et al.... 176/78 3,442,763 5/l969 Chetter et al176/78 3,604,100 9/1971 Tindale 29/433 3,646,994 3/1972 Piepers et al.176/78 UX 3,664,924 5/1972 Krawiec 176/78 3,679,546 7/1972 Muellner etal. 176/78 3,679,547 7/1972 Warberg 176/78 Primary Examiner(larlie T.Moon Attorney, Agent, or Firm-J. M. Maguife, Esq.; j P. Sinnott, Esq.

[5 7 ABSTRACT 'into the cell. The inserted cam is rotated to deflect thegrid surfaces that comprise the cell walls. This deflection prevents aleaf-spring mounted detent formed in the grid surface from extendinginto the path of the fuel rod. A key with a dimension that generallymatches the deflected grid is inserted into the cell in order to engagethe temporarily bowed grid surface. The inserted cam is rotated back tothe initial orientation relative to the cell configuration and iswithdrawn from the grid structure. The deflecting key lodged against thebowed grid surface, however, sustains the deflection in order to enablethe fuel rod to be inserted into the cell without being abraded by thedetent. After fuel rod insertion, the matching dimension portion of thedeflecting key is disengaged from the grid surface and the key iswithdrawn from the cell to release the leaf spring and allow the detentto clutch the fuel rod.

1 Claim, 5 Drawing Figures PATENTEnm 51914 sum 1 or 4 NVENTOR. FelixJabse AT RNEY PATENTEU 74 SHEET E OF 4 FIG.2

PATENTED 3. 795 .040

SHEEI It 0F 4 FIG. 5

GRID OPENING FIXTURE l1 HYDRAULIC 8 CYLINDER gs FLOW 85 /coNTRoI. 2

FOUR WAY 80 VALVE% METHOD OF INSERTING FUEL RODS INTO INDIVIDUAL CELLSIN A FUEL ELEMENT GRID CROSS REFERENCES TO RELATED APPLICATIONSCopending U. S. Pat. application Ser. No. 774,148, filed on Nov. 7,1968, now US. Pat. No. 3,665,586, by Felix S. Jabsen for Nuclear FuelRod Supporting Arrangements and a division of this application, U. S.Pat. application Ser. No. 105,388, filed Jan. 11, 1971 by Felix S.Jabsen for Nuclear Fuel Rod Supporting Arrangement, both assigned to theassignee of the instant invention.

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This inventionrelates to methods and apparatus for assembling nuclear reactor fuelelements, and more particularly to a cam and key system for flexing afuel element grid structure in order to facilitate fuel rod insertion,and the like.

2. DESCRIPTION OF THE PRIOR ART The concentration of fissionablematerial required for the operation of a nuclear reactor ordinarily isachieved through an array of fuel rods. These rods usually contain auranium compound in pellet form.

The structural integrity of these rods, however, presents a number ofvery difficult technical problems. For example, the nuclear reactionsthat occur within a reactor generate potentially dangerous quantities ofradioactive matter. Some fissionable materials, moreover, are highlytoxic. Consequently, if one or more of these rods should rupture and thewithin substances escape, a serious health hazard and a time-consumingand expensive decontamination and repair task will almost certainlyresult. The reactor environment makes the design of a sound fuel roddifficult because it imposes, for example, high temperature, coolantflow rate and pressure conditions as well as an intense radiationbackground. Accordingly, great care is exercised in the design,manufacture, and assembly of these fuel rods.

Fuel rods usually are assembled in clusters, or fuel elements" forinstallation as a part ofa nuclear reactor core. Some fuel elementdesigns engage the individual rods in a regular physical arrangement bymeans of grids. Typically, these grids are formed from a group ofinterlocking plates that establish a cellular structure, each cellhaving lodged therein a portion ofa fuel rod. Leaf springs, formed inthe plate surfaces are provided with detents that protrude into theindividual cells in order to contact and restrain the associated fuelrod.

The spring mounted detents present a manufacturing difficulty. If thedetents and associated leaf springs exert an adequate restraining force,these same detents necessarily must abrade the fuel rod as it isinserted into and passed through the cell in question. Fuel rod damageof this sort tends to hasten deterioration and reduce useful fuelelement life inasmuch, for example, as the scratches become corrosionloci.

There is a need to enable a grid structure of the foregoing sort toengage the rods in a fuel element with sufficient force to preventrelative movement between rod and grid without damaging the individualrods during fuel element assembly.

SUMMARY OF THE INVENTION To a large extent, the invention alleviates theproblem of fuel rod scoring during fuel element assembly. A cam, forexample, is inserted in a longitudinal direction into one of the cellsthat is formed by the interlocking grid plates.

Viewed in a plane that is transverse to this longitudinal direction, thecam preferably is square with rounded corners in general cross section.The transverse dimensions of the cam, moreover, are about the same sizeor slightly smaller than the projection onto a transverse plane ofasquare that is defined by the limits of the detents that protrude intothe cell from the grid plates. Rotating the cam about the longitudinalaxis causes the corners of the rounded cam to engage and press againstthe inwardly protruding detents. The force applied to these detents bythe rotated cam urges the detents in a direction that is outward fromthe center of the cell.

A key is inserted into the cell. One side of this key has a protrusionthat matches the depth of the grid deflection in order to engage thedeflected grid surface and temporarily sustain the deformation.

Preferably, a second key is inserted into the cell in a similar mannerand in a direction that is perpendicular to the direction in which thefirst key is inserted. A similar protrusion on the second key engageswith the adjacent deflected grid surface to maintain this deflection.With both keys lodged in the cell, the cam is rotated back to theinitial orientation with the transverse cross section of the camgenerally aligned with sides of the grid that form the cell. After thesecond rotation, the cam is withdrawn from the cell. The keys, however,continue to sustain the deflection in two of the grid plates that formsides of the cell. This sustained deflection establishes a sufficientclearance to enable a fuel element to pass longitudinally through thecell without being abraded or scored by the inwardly protruding detents.

After the fuel rod is positioned within the cell, the keys each arerotated to disengage the respective protrusions from the deflected gridsurfaces. The stresses that caused the grid deflections in this way arerelieved and force the associated detents inwardly toward the center ofthe cell. These detents bear against adjacent portions of the fuel rodsurface in order to clutch the rod between all of the cells inwardlyprotruding detents.

The rotated keys are withdrawn from the cell in a transverse directionto leave the fuel rod securely lodged within the grid structure.

More specifically, a typical grid structure may have in excess of twohundred cells formed by an array of grid plates that interlock at rightangles with each other. A depressing mechanism is provided to carry amatching number of retractors, or cams. These cams are inserted as agroup into the corresponding cells in the grid structure. Because thecams all are attached to the spring depressing mechanism support platethrough an array of cranks, or lever rod assemblies, an actuating platewhich gangs all of the cranks together for simultaneous movement impartsthe necessary degree of rotation to these cams.

The actuating plate, moreover, is driven by means of a hydraulic system.Typically, this system establishes a fluid pressure that is appliedselectively through a valve to one of two oppositely driven pistons,depending on the desired direction of cam rotation. The piston movementsare coupled to the actuating plate through a conventional piston rod andcrank arrangement.

In operation, the hydraulic system is activated to dirve the camsthrough an angular rotation of about 45. This movement enables therounded edges of the cams to engage the inwardly protruding leaf-springmounted detents and deflect these detents outwardly, away from thecenters of the respective cells. After the leaf springs have beendeflected by the cams, the keys are inserted into the cellularstructure. Preferably, each of these keys comprises a thin rod ofgenerally rectangular bar stock. The protrusions, one for each of thecells penetrated by the key, are formed by the extent of the longestside of the rectangle. To provide an adequate clearance for the fuel rodinsertion, however, some of the metal is removed from one side of thekey. The keys are inserted into the grid structure in a transversedirection through apertures formed at the perpendicular intersections ofthe grid plates. Thus, for example, there is one key for each plate thatforms the grid structure, each of these keys being generally parallel toand laying against a respective plate. At this stage in fuel elementassembly, portions of two keys are lodged in each cell. The longdimensions of each of the two keys in this instance being in engagementwith the respective deflected cell surfaces. These key portions crosseach other at a right angle in the common intersection that is formed bytwo of the grid plate surfaces.

After all of the keys have been positioned within the grid structure,the hydraulic system is manipulated to drive the cam actuating plate inthe opposite direction. This actuating plate movement restores the camsto their original orientations relative to the transverse cross sectionsof the respective cells. The grid structure, with the inserted keys nowsustaining the deformation, is withdrawn from the array of cams bymovement in a longitudinal direction.

It has been found preferable, when subjecting the grid structure to theoperation of the cams, to restrain the perimeter of the grid by meansofa production fixture or clamp. This restraint prevents the grid frombending or deflecting in some undesired direction in response to theforces applied by the cams.

While the keys sustain the deflection, the fuel rods are carefullyinserted into the individual cells to achieve the specific structuralrelations between the rods and the grid. The detents, deflected out ofthe path of the fuel rods as they are being inserted in respectivecells, are effectively restrained from scoring or abrading the rodsurfacesv When the fuel rods are lodged in place, the keys are rotatedto disengage the protrusions from the deflected detents. Thisdisengagement releases the detents and enables them to bear inwardlytoward the center of the respective cells and engage the adjacent fuelrod surfaces.

The rotated keys are withdrawn from the grid structure as hereinbeforedescribed to provide a fully assembled portion of a fuel element.

Thus there is provided in accordance with the inven tion an improvedtechnique for assembling fuel elements that reduces the risk of scoringor otherwise damaging the fuel rods during insertion into the grids.This system, moreover, eases and simplifies fuel element assemblyprocedures, thereby reducing costs and increasing production efficiency.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawing and descriptivematter in which there is illustrated and described a preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a prespective view of aportion of a grid structure showing a cam and key arrangement thatillustrates features of the invention;

FIG. 2 is a plan view in full section of a cell and cam combination inaccordance with the invention;

FIG. 3 is a plan view in full section of the cell and cam combinationshown in FIG. 2, with a different ori entation of the cam relative tothe cell;

FIG. 4 is a side elevation in broken section of a typi cal depressingmechanism assembly for use in connec tion with the invention; and

FIG. 5 is a schematic diagram of an hydraulic system for operating thedepressing mechanism shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a more completeappreciation of the invention, attention is invited to FIG. 1 whichshows a portion of a typical grid structure 10 that is formed by meansof an array of perpendicularly intersecting and interlocking grid platesl1, l2, l3, and 14. The surfaces of these plates, when joined, establisha group of cells of which only cell 15 is completely shown in FIG. 1.

The grid plates ll, l2, l3, and 14 are joined at respectiveintersections by means of spot welds (not shown). Cut out portions 16,I7, 20, and 21, provided in the central portions of the respectiveplates coincide at the grid intersections The surfaces of the gridplates that are bounded by the cut out portions, as for example, in theplate 11, are bent slightly to form a strip 22 and an adjoining strip 23by forming a transverse crease 24 that extends from the mid-point of thelongitudinal edge of the cut out portion I6 to the mid-point of thelongitudinal edge of the cut out portion I7. The crease 24 and theadjacent strips 22 and 23 combine to produce a shallow vee thatprotrudes away from the center of the cell 15 and toward the center ofan adjoining cell 25.

A protruding detent 26 is formed athwart the crease 24 at theapproximate middle of the crease between the cut out portions 16 and 17.The illustrative detent 26 is characterized by an inwardly juttinglongitudinal crest 27 that is generally perpendicular to the transverseorientation of the crease 24. This physical combination of the strips 22and 23, joined in a shallow vee at the crease 24 forms a leaf springthat resiliently supports the detent 26 for movement in the transverseplane. In a similar manner the cell 15 has leaf springs 30 and 31 formedin the surfaces of the mutually intersecting perpendicular grid plates13 and 14. The leaf springs 30 and 31 bear respective detents 32 and 33that protrude in toward the center of the cell 15, also for resilientmovement in the transverse plane.

The edge of the grid plate 12 also is provided with a detent 34. Adetent 35, moreover, is formed in the corresponding edge of theintersecting grid plate 11. Although the detents 34 and 35 protrudeinwardly toward the center of the cell 15, they are not mounted forresilient movement in the transverse direction. A detent 36 is formed inthe plate 11 in longitudinal alignment with the detents 35 and 26, buton the edge of the plate that is opposite to that edge which bears thedetent 35.

The grid plate 12 also has another detent formed in the transverse edgethat is in the same plane as the detent 36. The detent 36, moreover,does protrude inwardly toward the center of the cell 15.

In order to deflect the spring mounted detents 32 and 33 in a transversedirection away from the center of the cell 15, a key 37 is inserted intothe cell through the cut out portions 20 and 21. Preferably, the key 37is formed from a thin strip of bar stock that has a generallyrectangular cross section. The maximum dimension of the key crosssection, moreover, should not be greater than the depth to which thedetents 34, 35 and 36 protrude into the cell 15. As shown in FIG. 1 ofthe drawing, the key 37 extends in a transverse direction through thecell and is in engagement with the surface of the grid plate 14 thatforms one side of the cell 15.

A recess 40 is formed on one side of the key 37. Typically, the recess40 has the shape of a segment ofa circle. This shape can be manufacturedby grinding away portions of the stock from which the key is made. Inany event, the extent of the recess 40 should be ofa sufficicnt depth toprovide an adequate clearance for a fuel rod (not shown) when it isinserted into the cell 15.

The key urges the leaf spring 31 away from the center of the cell 15 tosustain an outward deflection of the shallow leaf spring vee that wasestablished initially in a manner to be described subsequently in moredetail. This key 37, moreover, imparts a slightly bowed form to the gridplate 14 that forms one side of the cell 15.

A key 41, with a recess 42 further engages a portion of the surface ofthe leaf spring 30 in order to deflect the leaf spring 30 and the detent32 formed thereon away from the center of the cell 15. In all respects,they key 41 that urges the leaf spring 30 away from the center of thecell 15 is similar to the key 37 and the leaf spring 31 described above.In the foregoing manner the keys press against the respective leafsprings 31 and 30 to sustain an initial deflection of these members awayfrom the center of the cell 15, as described subsequently in morecomplete detail.

In accordance with an aspect of the invention, before the keys 37 and 41are lodged in position within the cell 15, a retractor, or cam 43 isinserted into the cell to provide the initial deflections of the leafsprings 30 and 31. The cam 43, as shown in FIG. 2, comprises a length ofhardened drill rod steel of a generally square transverse cross section.The rod has four flattened sides of which adjacent sides 44 and 45 areillustrative. The corners of the cam 43 are, however, provided with around 46 on the corner that joins the adjacent flat sides 44 and 45.

The transverse distance between flat 45 and a parallel flat 47 isslightly less than the distance projected into a transverse plane thatseparates the crests of the detents 32 and 34 which protrude into thecell 15 when the leaf spring 30 that supports the detent 32 is notdeflected.

In contrast, diagonal distance 52 between the round 46 and oppositeround 50 is generally equal to the transverse distance between theopposing parallel surfaces of the grid plates 12 and 13, and exclusiveof the depth to which the leaf spring 30 protrudes into the cell 35.

The spring retractor or cam 43 is inserted into and centered within thecell 15. To deflect, or to bow out the leaf springs 30 and 31 from thesides of the grids 13 and 14, the cam 43 is rotated in either aclockwise or in a counter clockwise direction as shown by a doubleheaded arrow 53.

By continuing, for example, a clockwise rotation through an angle ofabout 45 as viewed in the plane of the drawing, the round 46 and a round55 engage the crests of the leaf spring mounted detents 32 and 33. Thisengagement urges and deflects these detents away from the center of thecell 15. At the same time, recesses machined into the round 46 and acomplementary round 55 on the cam 43 provide a passageway thatfacilitiates the insertion and withdrawal of the keys that sustain thedetent deflection.

The leaf springs 30 and 31 are deflected by the cam 43 (FIG. 3) awayfrom the center of the cell 15 to force the crests of the springs toalign generally with the respective surfaces of the grid plates 13 and14 that form two sides of the cell 15. The clearance between thesedeflected sides and the recesses that are machined into the adjacentrounds on the cam 43 is sufficient to enable the associated keys 41 and37 (FIG. 1) to be lodged in the cell 15.

Thus, as shown in FIG. 3, this clearance is wide enough to enable therespective keys 37 and 41 to pass between gaps 57 and 60 establishedbetween the cam 43 and the temporarily deflected leaf springs 30 and 31.

Turning once more to FIG. 1, the keys 37 and 41 press against therespective adjacent leaf springs and maintain this leaf springdeflection. Thus, the cam 43 is rotated in a direction opposite to thatin which it originally was shifted to produce the preferred deflection.After rotation, the cam 43 assumes the orientation shown in FIG. 2 anddisengages the rounds 46, 50, 54, and 55 from the surfaces that comprisethe cell 15. The cam 43 is withdrawn from the cell 15 by moving itlongitudinally, in a direction that is perpendicular to the plane of theFIG. 2 drawing. The cam 43 illustrated in FIG. 1 is withdrawn from thecell 15, and the keys 37 and 41 sustain the temporary outwarddeflections of the cell surfaces, or more specifically, the deflectionsof the leaf springs 30 and 31 in the grid plates 13 and 14 that form apart of the bounds of the cell 15.

This apparently small deflection, nevertheless, enables a fuel rod (notshown) to be inserted into the cell 15 without scraping against thedetents that protrude into the center of the cell. The initialdeflection by means of the cam 43, moreover, relieves the slender keys37 and 41 of the burden of producing the leaf spring deformation. Thispreferred procedure reduces the incidence of broken and twisted keysthat heretofore have, on occasion, marred attempts to produce theinitial deflection by turning the keys to urge the key protrusionsagainst the respective leaf springs.

When the fuel rod is properly positioned within the grid structure 10,the keys 37 and 41 are rotated in the direction shown by arrows 61 and62 in FIG. 1 t0 disen-. gage the broad dimension of the keys crosssection from the leaf spring surfaces of the grid plates 14 and 13,respectively. The elasticity inherent in the leaf springs 31 and 30 thencauses these springs to shift in toward the center of the cell 15. Inthis condition, the spring mounted detents 32 and 33 are pressed againstadjacent portions of the fuel rod surface, and in this way to firmlyclutch the fuel rod with all of the detents that protrude into the cell15. For the purpose of simplified description, the invention thus farhas been disclosed with respect to one cell and one fuel rod. The usualindustrial grid structure, however, may accommodate a group of severalhundred fuel rods. Thus, in accordance with another feature of theinvention, the cams are arranged in a ganged array to facilitate gridloading in a minimum number of operations. As shown in FIG. 4, forexample, an array of two hundred and eight cams depend from a depressingmechanism assembly FIG. 4 of the drawing illustrates a typical depressing mechanism.

An individual cam in the array 63, of which the cam 65 is typical, isconnected to the assembly 64 through individual roll pins 66 that/aretransverse to the longitudinal cam axis. The roll pin 66 connects thecam 65 to an individual lever rod assembly 67. The end of the lever rodassembly 67 that protrudes from the depressing assembly is nested intelescopic engagement within a central bore (not shown) that is formedwithin the cam 65. The middle portion of the lever rod assembly 67 ischaracterized by a crank 70. An actuating plate 71 pivots the crank 70to translate a reciprocating plate motion into a rotation of the cam 65in the transverse plane. All of the cams in the array 63 are similarlyganged to respond to the movement of the plate 71.

Illustrative of all of the cranks, the crank 70 has retaining rings 72and 73 on opposite sides of an aperture 74 which is formed in the plate71. The end of the lever rod assembly 67 which is opposite to the endthat is coupled to the cam 65 is pivotally received in a support plate75. A shank on the lever rod assembly adjacent to the cam 65, moreover,is pivotally mounted in another support plate 76. The end of the rodassembly 67 that is pivotally received in the plate 75 also is providedwith a retaining ring 77.

To rotate the cam 43 through an angle of, for example, 45 as shown inFIGS. 2 and 3, the actuating plate 71 is shifted arcuately in thetransverse plane. This movement drives the individual crank portions ofthe respective lever rod assemblies through circular arcs that subtendabout 45. The shank portions of these lever rod assemblies, as well asthe cams connected thereto. consequently rotate through the same angle.

FIG. shows an hydraulic system 80 that provides the forces needed toshift the actuating plate 71 in two opposite directions through theappropriate distance. The actuating plate 71, for instance, is connectedthrough a linkage and a piston rod 81 to a piston (not shown) in anhydraulic cylinder 82 in order to shift the plate into one of these twopredetermined positions. The fluid pressure that drives the piston inthe cylinder 82 is provided through a conduit 83, which is con nected toan outlet in a four-way control valve 84. The movement imparted to thearray of cams through this shift in the position of the plate 71produces a shift in cam position that corresponds to a rotation from anorientation characterized by a general alignment with the sides of thecell (FIG. 2) to an engagement between the rounds on the cams and therespective spring mounted detents (FIG. 3).

The return stroke, which rotates the array of cams back into theinitial, aligned orientation relative to the sides of the individualcells, is provided through a piston rod and linkage 85 that is coupledto a piston (not shown) within an hydraulic cylinder 86. Fluid pressurefor the cylinder 86 is applied through a conduit 87 from another of theoutlets on the control valve 84. The hydraulic fluid is supplied to thevalve 84 through a conduit 90 that connects the discharge outlet of ahydraulic pump 91 to an inlet on the control valve 84. The fluid returnfor the hydraulic system from the valve 84 to an inlet on the pump 91 iscompleted through a conduit 92. A throttle valve or flow control device93 and a gate valve 94 also are coupled to the conduit 91 in order toregulate the hydraulic fluid in the system.

In operation, FIG. 2 shows the cam 43, which is one of the array ofearns 63 (FIG. 4) that are inserted into the cells in the gridstructure. To prevent the move ment of the cams from warping or twistingthe grid structure 10, a clamp (not shown), rigidly engages the gridperiphery and locks the grid in position. The clamped grid is slippedover the array of cams 63 (FIG. 4), each cell in the grid structurereceiving a respective cam. As hereinbefore mentioned, the flat surfacesof the cam are in general parallel alignment with the sides of the gridthat form the cell boundaries.

The control valve 84 (FIG. 5) is manually operated to couple hydraulicpressure from the pump 91 through the conduits 90 and 83 to thehydraulic cylinder 82. The piston rod and linkage 81 respond to theapplication of this pressure by shifting the position of the actuatingplate 71 and thus, the cranks that are ganged to this plate move througha circular arc of predetermined distance. As shown in FIGS. 2, 3, and 4,this motion is translated into a rotation in the transverse plane of thecams that are connected to the respective lever rod assemblies in thespring depressing mechanism 64. This rotation of the cams, through auniform angle of about 45, produce an initial outward deflection in theleaf springs that heretofore had protruded into the individual cells asthe rounds on the cams engage and press against the adjacent detents onthese springs.

Turning now to FIG. 1, the keys 37 and 41 are inserted through the cutout apertures 20, 21, and 17, 20 respectively that are formed at thecell corners. The broad dimension of the cross section of the keys 37and 41 bear against the respective deflected leaf springs 31 and 30, themaximum transverse extent of this dimension being approximately equal tothe depth of the leaf spring deflection. After lodging the keys 37 and41 within the cell 15, the valve 84 (FIG. 5) is once more operated. Inthis condition, the valve 84 relieves the hydraulic pressure in thecylinder 82 and applies hydraulic pressure to the return cylinder 86 inorder to force the piston rod and linkage 85 to drive the actuatingplate 71 through the same circular arc, albeit in a direc tion oppositeto that in which it was driven by the piston rod and linkage 81. Thisreturn stroke pivots the ganged cams back to the initial insertionorientation relative to the portions of the grid sides that form theindividual cells in the grid structure. As shown in FIG. 1, the cam 43is withdrawn in a longitudinal direction from the associated cell 15 toillustrate the manner in which all of the cams in the grid structure areremoved from the depressing mechanism assembly. The keys 37 and 41retain the respective leaf springs 31 and in their outwardly bowed ordeflected position. The fuel rods are inserted into the temporarilyenlarged apertures that characterize the cells in the grid structure 10.The bowing imparted to the deflected leaf springs that form part of thesides of the individual cells provide a sufficient clearance to enablethe fuel rods that are being inserted to avoid a scraping or gougingcontact with the detents that obtrude into the cells in the gridstructure.

After the fuel rods each are received within the individual cells, thekeys 37 and 41 are manually rotated in the direction shown by the arrows61 and 62 in FIG. 1 to shift the long cross section dimension of thekeys away from the respective deflected leaf spring surfaces. Turningthe broad dimension of the keys away from the leaf springs, relieves thestresses on the keys 37 and 41. Because the keys are being turned to acondition of lower stresses, the possibility of twisting or breaking thekeys during this step in the fuel element assembly is quite small. Therotated keys are withdrawn from the grid structure through the cutoutapertures 20, 21, and 17, 20. Rotating the keys 37 and 41 in this manneralso relieves the deflecting forces on the leaf springs 31 and 30. Theinherent resiliency of these springs causes them to drive the respectivedetents against the outer surface of the fuel rod (not shown) andthereby clutch the rod between all of the detents that protrude inwardlytoward the center of the cell 15.

A preferred embodiment of the invention does not require the leafsprings 30 and 31 (FIG. 1). For example, the detents that protrude intoward the centers of the respective cells can be formed in the flatsurfaces of the individual grid plates. Cams, operated in the mannerdescribed above, urge or deflect the detents away from the centers ofthe respective cells. It has been found that the depths of thedeflections that are provided by these cams are sufficient to enable thekeys to be inserted into the cells. After the cams are withdrawn fromthe grid structure the deflections in the grid piates continue to besustained by the keys. The fuel rods are positioned in the cells and thekeys are then rotated to disengage from the surfaces of the plates. Asthese keys are being rotated, the inherent resiliency of the grid platestends to urge the detents back in toward the centers of the respectivecells. Thus impelled, the detents clutch the adjacent surfaces of therespective fuel rods in order to retain the rods in the predeterminedpositions within the grid structure.

Thus, there is provided in accordance with the invention an improvedmethod and apparatus for assembling fuel elements that protects the fuelrods from scoring and abrasion during assembly.

I claim:

1. A method for inserting fuel rods into individual cells formed in afuel element grid that has detents that protrude in toward the center ofthe cell comprising the steps of inserting a cam into the cell in onedirection, rotating the cam in a plane that is generally transverse tosaid insertion direction, engaging a surface of the cam against the atleast one detent, deflecting said detent in a direction that is awayfrom the center of the cell, inserting a key into the cell, engaging asurface of said key to sustain said deflection, withdrawing the cam fromthe cell, inserting a fuel rod in the cell, and disengaging the key fromthe cell to relieve said deflection and enable the detents to clutch thefuel rod lodged in

1. A method for inserting fuel rods into individual cells formed in afuel element grid that has detents that protrude in toward the center ofthe cell comprising the steps of inserting a cam into the cell in onedirection, rotating the cam in a plane that is generally transverse tosaid insertion direction, engaging a surface of the cam against the atleast one detent, deflecting said detent in a direction that is awayfrom the center of the cell, inserting a key into the cell, engaging asurface of said key to sustain said deflection, withdrawing the cam fromthe cell, inserting a fuel rod in the cell, and disengaging the key fromthe cell to relieve said deflection and enable the detents to clutch thefuel rod lodged in the cell.